Methods of forming images by heat development are described, e.g., in U.S. Pat. Nos. 3,152,904 and 3,457,075, and D. Klosterboer, Thermally Processed Silver Systems (chap. 9, p. 279, in the book entitled “Imaging Processes and Materials”, compiled by J. Sturge, V. Walworth & A. Shepp, 8th ed., published by Neblette in 1989). In general a heat-developable photosensitive material contains a reducible light-insensitive silver source (e.g., an organic silver salt), a catalytically active amount of photocatalyst (e.g., silver halide) and a silver-reducing agent in an organic binder matrix-dispersed condition. Such a heat-developable photosensitive material is stable at room temperature, but it produces silver through an oxidation-reduction reaction between a reducible silver source (functioning as an oxidizing agent) and a reducing agent when heated at a high temperature (e.g., at least 80° C.) after exposure. This oxidation-reduction reaction is accelerated by a catalytic action of latent images formed under exposure. The reducible silver salt produces silver through the reaction in exposed areas, and thereby the exposed areas are blackened and stand in contrast to unexposed areas. Thus image formation is effected.
On the other hand, the most common method of color-image formation for photographic light-sensitive materials is a method of utilizing coupling reaction between a coupler and an oxidized color developing agent. JP-A-9-10506 (the term “JP-A” as used herein refers to an “unexamined published Japanese patent application”) and European Patent No. 762,201 disclose methods of the type which forms color images in a photosensitive material by supplying a small amount of water to a light-sensitive element wherein a developing agent and couplers are incorporated, pasting the light-sensitive element to an image-receiving element containing a base precursor, and then heating these elements to cause development reaction therein. In addition, U.S. Pat. Nos. 3,761,270, 4,021,240, 4,426,441 and 4,435,499, JP-A-59-231539 and JP-A-60-128438 disclose the heat-developable color photosensitive materials in which image formation is effected by heating treatment alone and does not require such a complex constitution as to include incorporation of a base precursor and supply of a small amount of water. In those patents, p-sulfonamidophenol, ureidoaniline and sulfonylhydrazone are used as color developing agents. The photosensitive materials utilizing the coupling system have a sensitivity advantage because couplers have no absorption in the visible region before processing, and it is a considerable point in their favor that they can be used as not only printing materials but also picture-taking materials.
Moreover, JP-A-9-204031, JP-A-2000-171914, JP-A-2000-40590 and JP-A-2002-169233 propose systems of the type which includes forming color images on a picture-taking sensitive material, reading the color images at once with a scanner to digitize them and printing positive images onto another material by using the digitized image information.
When the heat-developable photosensitive materials are used as picture-taking sensitive materials in those simple-and-rapid negative printing systems, the processing is simple and rapid, and besides, it requires no complicated management of processing solutions, compared with conventional picture-taking materials. In addition, the heat-developable photosensitive materials enable simplification and miniaturization of processing machines, so have an advantage of permitting processing machines to be placed anywhere. Therefore, realization of heat-developable color photosensitive materials ensuring high-quality images has been desired.
In many cases, thermal solvents are used in those heat-developable photosensitive materials for the purpose of increasing developed-color densities through improvements in thermal decomposition rates of precursors of color developing agents and mobility of incorporated color developing agents. It is thought that, when developing the photosensitive materials by heating, the precursors of color developing agents are dissolved in the fused thermal solvents and the thermal decomposition rates thereof are increased, and besides, the color developing agents released from the precursors are also dissolved in the fused thermal solvents and their diffusion in binder matrices are speeded up. However, there is apprehension that color images spread and become blurred at the time of heating because couplers and dyes produced by reaction between the couplers and the oxidized color developing agents are also dissolved in the fused thermal solvents. It is therefore important in preventing the spreading of dye images to search thermal solvents having high compatibility with color developing agents as well as their precursors and low compatibility with couplers and color-developed dyes. Hitherto, thermal solvents have been studied mainly to assess their effects in applying them to the methods of positively diffusing and transferring dyes formed or released by development reaction in forming color images, as in U.S. Pat. No. 5,843,618, JP-B-6-82209 (the term “JP-B” as used herein refers to an “examined published Japanese patent application”) and Japanese Patent Nos. 2,700,808, 2,711,339 and 2,711,340. On the other hand, there has been known practically no study on suitability of thermal solvents in the cases of using heat-developable photosensitive materials as picture-taking materials. Therefore, there is little information as to thermal solvents that have high development activity and hardly cause the spreading of dyes by diffusion, so the problem of blurred color images is difficult to solve by use of previous knowledge. Under these circumstances, simple alternative proposals have been sought.
Further, as described in the patents cited above, it is common knowledge in the field that the selection of what kind of thermal solvent to use affects greatly the raw-stock storability of a photosensitive material. And the above-cited patents also have a description that the raw-stock storability, especially the raw-stock storability under high-humidity conditions, is higher in the case of using a lipophilic, solid dispersion-capable thermal solvent. However, the inventors' study has revealed that the lopophilic, solid dispersion-capable thermal solvents were apt to cause spreading of dyes because of their high compatibility with lipophilic couplers and dyes formed by reaction between lipophilic couplers and oxidized color developing agents.
When heat-developable photosensitive materials are used for picture-taking purpose, the spreading of dyes causes serious deterioration in sharpness of prints, and so it is in need of improvement.